This invention relates to an electric motor and in particular to a permanent magnet rotor for an electric motor.
Permanent magnet rotors are well known and are used, for example, in brushless D.C. motors and sychronous motors. Various methods have been tried to attach the magnets to the rotor. A common construction is to use a ring magnet rather than individual bar magnets to overcome centrifugal forces. These are usually sintered magnets formed as simple rings with ground internal and external surfaces to provide accurate diameters.
The return path for the magnetic field in the rotor is provided by a flux ring made from an iron tube, a stamped stack of steel laminations or a solid ring. The flux ring needs to have a highly accurate outside diameter to match the inside diameter of the magnet as the magnet is a press fit on the flux ring. If the interference is too great, the magnet may crack. If the interference is too little, the magnet may be loose on the flux ring resulting in slippage under load.
It is known to use a plastic coupler to connect the flux ring to the motor shaft. The coupler is usually in the form of a solid sleeve. Again, the external diameter of the coupler needs to be accurate to ensure a good fit and reliable transfer of torque between the magnet and the shaft. This is a particular problem when the motor is designed or required to operate over a wide temperature range due to the differences in thermal expansion of the various parts.